Lithium micro-batteries are well known today. They conventionally comprise a substrate on which a stack is deposited comprising successively a cathode, an electrolyte comprising lithium, and an anode generally made of metallic lithium, the unit so formed being covered by a protective housing in order to prevent any contamination from outside.
These micro-batteries, implemented in the form of thin films using microelectronics technology, are therefore implemented using lithium-based materials, which are especially sensitive to the external environment, and above all to moisture, and to the oxygen and nitrogen present in the ambient air. The protection they are given against these different agents is the decisive factor in respect of their durability.
The operating principle of these cells hinges on the insertion and disinsertion (or intercalation-disintercalation) of an alkali metal and typically lithium ion into the positive electrode. The principal micro-battery systems use the lithium ion Li+ from a metallic lithium electrode as the ion species. The different components of such a micro-battery, whether it be the current collectors, the positive or negative electrodes, the electrolyte or the protective layer also known as the encapsulation layer, are presented in the form of thin layers obtained by Physical Vapor Deposition (PVD) or Chemical Vapor Deposition (CVD). The total thickness of the stack is typically about 15 μm.
The nature of these different components may be as follows:
The collectors: these are metallic in nature, constituted for example of platinum, chromium, gold, titanium, tungsten or molybdenum.
The positive electrode: this may be made of LiCoO2, LiNiO2, LiMn2O4, CuS, CuS2, WOySz, TiOySz, V2O5. Depending on the material selected, a thermal annealing operation may prove necessary in order to improve film crystallization and their insertion property within the stack. This is especially true of lithium oxides. However, some amorphous materials, and particularly titanium oxysulfides (TiOySz), do not need to be treated in this way while allowing a high level of Li+ lithium ion insertion.
The electrolyte: this must be a good ion conductor but an electronic insulator. Most of the time, a vitreous material is employed based on boron oxide, lithium oxide or lithium salt. The most effective electrolytes are phosphate based, and especially LIPON (lithium compound based on lithium phosphorus oxynitride) or LISIPON (lithium compound based on lithium phosphorus and silicon oxynitride);
The negative electrode: this must be constituted of metallic lithium deposited by thermal evaporation, or a lithium-based metal alloy or again an insertion compound (SiTON, SnNx, InNx, SnO2 etc.).
The encapsulation layer: the purpose of this, as already stated, is to protect the active stack constituting the micro-battery from the external environment and specifically from moisture.
This encapsulation may be the outcome of two different technologies:
Under a first technology, this encapsulation is implemented on the basis of thin layers. A stack of layers is thus generally employed in a vertical alternation of different layers in order to optimize the barrier properties of the unit. The most commonly adopted strategy involves depositing a pre-encapsulation by means of a layer that is inert as regards lithium and planarizing. The conventionally used material is a polymer, in the case in point parylene. The barrier function may be reinforced through the deposition of other layers, in particular metals or organosilicons.
This technology comes up against a problem of limited durability. Indeed, because of the permeation of gases through the encapsulation so implemented, as well as the mechanical stresses sustained by the encapsulation during the electrochemical cycling of the micro-batteries (volume changes), the lithium is affected, and it oxidizes quite quickly as a result.
Under a second technology, encapsulation is implemented by capping: a layer of epoxy is deposited on the edge of the element to be protected. This allows a glass cap to be bonded by exposure to ultraviolet radiation. The cap-to-substrate seal can be implemented using a number of methods and may involve the use of different materials, either metals, or dielectrics.
It can be seen therefore that this technology requires the deposition of a sealing layer to anchor the cap onto the substrate. This layer is not however generally optimized in respect of barrier properties, particularly as regards moisture but also oxygen and nitrogen. Moreover, the seal in use comes up against outflow problems in relation to the current collectors from the cavity comprising the micro-battery. In fact a non-metallic seal has to be used, at the very least in proximity to the collectors in order to prevent short-circuiting. Yet metallic materials have proved to be the materials that have the best barrier effect with the result that the problem can easily be imagined, in so far as it is necessary to make do with an unsatisfactory barrier effect. In fact, the use of a metallic seal is only possible in the event of the micro-battery being connected vertically, in other words through the rear surface of the substrate. But this solution requires additional technological steps that are highly complex to apply, and thereby raise the implementation costs.
The objective set by the present invention is to provide an encapsulation for such micro-batteries that is not only efficient in terms of barrier effect, but also employs a widely mastered implementation technology and especially requires no additional steps for the deposition of a seal-specific material.
The invention implements the technology of encapsulation by capping.